Field of the Invention
The present invention includes a method and apparatus for preventing extraneous detection of signal information from raster scan signals in a plurality of consecutive line signals intended to form an image or partial images.
Discussion of Related Art
Raster scan signals may be divided into video, telefax and laser print signals. Common to all of these signal types is that an image or text page is built up from a plurality of consecutive lines, the least element of which comprises image dots, i.e. pixels. A special case of such raster scanned signals is video signals, the latter also having, apart from the consecutive lines, a consecutively repeated image field. The raster scanned signals result in high information redundancy.
Raster scanned signals in display screens, laser printers, telefaxes and other computer and IT equipment contain radio frequency components, which are radiated to the surroundings and are propagated through the air and via conductors connected to the object in question, or situated close to it. The information content in these signals can be intercepted and interpreted remotely, without the user noticing anything, which is a great hazard to data security.
A previously known method of reducing the emission of these information signals is to encapsulate and screen the equipment, so that only negligible signal levels reach the surroundings. This technique is however only utilisable for newly manufactured equipment, and is generally very expensive, both in production and maintenance.
It is also previously known to emit around the equipment an interference signal having the characteristic of white noise. This technique is also burdened with considerable drawbacks. The protective effect that can be achieved by this method is thus limited, since the collective mean value of white noise is zero the mean value formation will reduce the protective effect obtained.
A still further known method of protecting signal information is to transmit a bit stream sequence of pseudo-random character, the clock signals thereof being of the same order of magnitude as the display screen pixel clock frequency but frequency modulated, see SE,B, 8900288-5. There are weaknesses in the protection obtained by this method as well, as signal processing with mean value formation is used.